1. Field of the Invention
The present invention relates to transmission of digital information in disturbed media and is concerned with a clock-frequency recovery device employed in equipment for the transmission of digital information between two terminal stations whether or not under conditions of visibility, the communication link being established in the majority of instances by tropospheric or ionospheric propagation.
Communication links of this type exhibit a propagation loss which is highly variable as a function of time. In order to ensure continuous transmission of information, it is not economically feasible and is even practically impossible in many cases to utilize equipment having a sufficient transmission margin to ensure a given minimum quality of communication in the event of occurrence of maximum propagation loss during a small percentage of time.
2. Description of the Prior Art
The expedient adopted in order to solve this difficulty consists in making use of a plurality of transmission channels in parallel, the characteristics of said channels being sufficiently different to ensure that they are not simultaneously affected by the same attenuation, or propagation loss. This has the effect of producing diversity operation in which the signals of the different channels are combined whilst weighting functions are automatically adjusted in order to obtain at each instant a quality at least equal to that of the channel which exhibits the best quality.
Since the diversity order employed is limited, there remain difficult periods of propagation, especially under conditions of tropospheric or ionospheric scatter propagation during which the demodulated and combined signal is of highly impaired quality. During short time intervals (which are subject to variation within the range of a few milliseconds to a few seconds), the signal-to-noise ratio obtained is very low and may even tend towards zero.
In point of fact, however, the demodulated and combined signal of the analog type must be regenerated in digital form. To this end, said signal is switched to two channels:
a clock-frequency recovery channel, PA1 and a regeneration channel in which regeneration is carried out by means of said recovered clock frequency. PA1 the digital signal train containing the information, PA1 the clock signal in phase with said digital signal train. PA1 the recovered clock frequency exhibits negligible phase jitter; PA1 maintenance of the bit clock signal in the presence of interruptions of the receiver signal can reach several seconds, the resultant phase-shift being less than 1/4 of a bit in order to ensure that said signal remains usable and; PA1 return to the initial phase at the instant of reappearance of the received signal is practically instantaneous.
After regeneration, the transmission equipment restitutes the two signals which it has the function of transmitting, viz:
These two signals are processed by the digital processing equipment (multiplexer, computer, and so on), the clock being usually employed for synchronization of the digital process.
Good operation of these processing equipment units is characterized by maintenance of their synchronism and will therefore depend on the quality and maintenance of the bit clock signal in the presence of severe fade-out caused by propagation.
These fading variations, or fade-out, result in the appearance of phase jitter in the bit clock signal which consequently becomes unusable beyond a certain value.
The clock-frequency recovery devices usually employed are not optimized for operation with a very low signal-to-noise ratio. Furthermore, the circuits employed frequently exhibit a threshold effect in the presence of high noise.